MLA

Pound, Ezra, 1885-1972. Bacon, Frank. Box 2, folder 34. n.d.. https://collections.library.yale.edu/catalog/32308883.

APA, 6th edition

Pound, Ezra, 1885-1972. (["Box 2, folder 34"])(n.d.)[Bacon, Frank]. https://collections.library.yale.edu/catalog/32308883.